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The cross-industry Electricity Network Strategy Group (ENSG) is proposing strengthening the grid network to allow thousands of megawatts of renewables-generated electricity - mostly wind - to be connected. These reinforcements include offshore subsea cables, dubbed bootstraps, along the east and west coasts of Scotland to transport electricity from the north of Scotland and the isles to areas of electricity demand further south, avoiding the onshore grid bottlenecks.

But National Grid says the proposals could push up Scotland's already high charges for generators to use the transmission system. In the north of Scotland these are as much as £20/kW of capacity per year, compared with £12.50/kW in the south of Scotland or just £0.81/kW for south-east England. National Grid estimates ENSG's proposed reinforcements could push charges up to £50/kWh.

"No one knows how the reinforcements are going to be paid for," says Callum McCallum, director of business development at Scottish Renewables. "Who bears the cost - is it the Scottish generators or is it consumers in the north of England?"

Transmission charging

National Grid has agreed to consider a different methodology for transmission charging which would be based on generators' actual use of the network, rather than the capacity of their plant. This would particularly help variable renewables - such as wind generators - who seldom use their connection at 100% of its rated capacity.

Changing to a charging system based on generation raises questions about balancing the system, McCallum adds. "You've got a wind generator on one transmission line which you only use one third of the time," he says. "You could put something else on that line for the other two-thirds of the time."

This smarter use of the system could mean that some of the planned additional grid infrastructure need not be built, McCallum says. "It's what connect and manage is all about," he says, referring to the measure that has been introduced in parts of the country to allow wind to connect to the system before new reinforcements are built.

But it could be years before any change is made. National Grid say it requires at least another two years of modelling before it can produce a formal proposal.